CD-1 mice were exposed to NO2 and morpholine, NO2 alone, morpholine alone, or neither, intermittently for 30 wk. The incidence of pulmonary adenomas detected during the following 2 yr was significantly higher in the combination group than in the other treatment groups or controls. Work is continuing in an attempt to determine the mechanism of the in vivo formation of a putative carcinogen, possibly N-nitrosomorpholine. Strain A/J mice were found to develop pulmonary adenomas in numbers that were a function of dose of urethane and time after exposure. Doses ranged from 250 to 1000 mg/kg. Lungs were examined 4, 5 and 6 months after injection. Urethane is used as a positive control carcinogen.